The present invention relates to a method and system for the simultaneous measurement of strain and temperature utilizing principles associated with Brillouin scattering.
Brillouin scattering is an inelastic or nonlinear scattering of light from acoustic phonons in a dielectric material, such as an optical fiber. Brillouin scattering can be spontaneous, as when light in a fiber interacts with density variations in the fiber, or it can be stimulated. The Brillouin frequency is the difference between the frequencies of the input and scattered beams of light within the fiber. The Brillouin frequency can be described by the equation:
                              V          B                =                                            2              ⁢                                                          ⁢                              nV                a                                                    λ              p                                ⁢          sin          ⁢                      θ            2                                              (        1        )            where: Va is the sound velocity in the optical fiber;
n is the refractive index;
λp is the wavelength of the pump laser.
The Brillouin frequency is a physical property that is related to temperature and strain within the optical fiber, in accordance with the following equation:VB=VB0+CT(T−T0)+Cε(ε−ε0)  (2)where CT and Cε are coefficients of temperature (T) and strain (ε), respectively. These coefficients are determined experimentally for each fiber.
With Brillouin amplification, the scattered light is amplified. There can be an energy exchange between two counter-propagating laser beams, which exchange is maximum when V1−V2=VB.
The Brillouin frequency spectrum is obtained by scanning the beat frequency of the fiber. It is characterized by the peak power, the shape of the frequency curve, the center frequency, and the linewidth, with full linewidth occurring at half-maximum (see FIG. 1).
It has been known that the principles of Brillouin scattering can be used to measure strain or temperature in an optical fiber. Because there is only one peak of a Brillouin spectrum from a single mode fiber (eg. SMF-28) and because strain and temperature change simultaneously in accordance with equation 2, it is impossible to simultaneously extract information respecting both strain and temperature from a single peak of the Brillouin spectrum.
In the past, when is has been desired to measure both strain and temperature simultaneously, it has been necessary to take special measures to achieve these measurements. For example, if temperature is maintained constant it is possible to measure strain, or if the strain is maintained constant it is possible to measure temperature. Another measure would be to install an additional fiber for temperature measurement in order to compensate for the temperature influence on the Brillouin spectrum caused by both temperature and strain. One then could measure both the Brillouin frequency and the intensity of the Brillouin spectrum. Alternatively, one can use special fibers, such as photonic crystal fiber (PCF), or large effective area fiber (LEAF) as the sensing media. FIG. 2 shows simultaneous measurement of strain and temperature using PCF and LEAF.
FIG. 3 shows the effect of temperature with such measurements, where it is seen that the central frequencies of the peaks at a and c increased linearly with temperature. The temperature coefficients are 0.96 for peak a and 1.23 MHz/° C. for peak c at 1320 nm. The pulse width was 1.5 ns˜15 cm spatial resolution.
FIG. 4 shows the effect of strain with such measurements, where it is seen that the Brillouin frequencies of peaks a and c have a linear dependence on the strain. The strain coefficients are 4.78×10−2 for peak a and 5.5×10−2 MHz/με for peak c at 1320 nm. The pulse width was 1.5 ns˜15 cm spatial resolution.
There are disadvantages to using PCF or LEAF for simultaneous measurement of strain and temperature. In real-life applications, peak c is easily covered by the noise resulting in a low signal to noise ratio. The intensity of the peak may vary greatly because of tension or compression in the fiber. In order to increase the spatial resolution, an increased baseline for the input pulses may be required, resulting in a complication of the Brillouin spectrum, and increased difficulties in identifying peak c.
There is therefore a need to devise a method and a system for the simultaneous measurement of strain and temperature in an optical fiber, and which does not suffer from the drawbacks associated with present methods and systems.